I did a cruise to Vietnam in 2001. Danang was one of our stops. I'm not familiar with Hue. We did a tour that involved seeing the sights and a stop for a drink and time at China Beach.
If your itinerary involves Ho Chi Minh City, I would recommend the Chu Chi Tunnels.

The closest I got to Vietnam was the Philippines in the Marine days, although when I got transferred to Philadelphia I befriended some Vietnamese refugees. "Ma" spoke no English but was a great cook. She had owned a bar and restaurant in Saigon. Man, that was usually some pretty spicy food! Pho - love it.

Some cruise ships that show Da Nang on their itinerary actually dock at the port of Chan May just under an hour from Da Nang. So you need to check if your ship is one that docks in Da Nang itself. (I think Royal Caribbean uses the port at Chan May.)
If you get immigration forms to fill out, don't lose them as they are collected on return to the ship.
I've been looking at the possibility of having a holiday/cruise to Vietnam and I think I would prefer Hue to Da Nang.

Jim C even though it was 10 years ago what was your impression of Da Nang ?